Various expansion processes are known for hydrocarbon liquids recovery, especially in the recovery of ethane and propane from high pressure feed gas. Most of the conventional processes require propane refrigeration for feed gas chilling and/or reflux condensing in the demethanizer and/or demethanizer, and where feed gas pressure is low or contains significant quantity of propane and heavier components, demand for propane refrigeration is often substantial, adding significant expense to the NGL recovery process.
To reduce external propane refrigeration requirements, the feed gas can be cooled and partially condensed by heat exchange with the demethanizer overhead vapor, side reboilers, and supplemental external propane refrigeration. The so formed liquid portion of the feed gas is then separated from the vapor portion, which is split in many instances into two portions. One portion is further chilled and fed to the upper section of the demethanizer while the other portion is letdown in pressure in a single turbo-expander and fed to the mid section of the demethanizer. While such configurations are often economical and effective for feed gas with relatively high C3+ (e.g., greater than 3 mol %) content, and feed gas pressure of about 1000 psig or less, they are generally not energy efficient for low C3+ content (e.g., equal or less than 3 mol %, and more typically less than 1 mol %), and particularly where the feed gas has a relatively high pressure (e.g. 1400 psig and higher).
Unfortunately, in many known expander processes, residue gas from the fractionation column still contains significant amounts of ethane and propane that could be recovered if chilled to an even lower temperature, or subjected to another rectification stage. Most commonly, lower temperatures can be achieved by high expansion ratios across the turbo-expander. Alternatively, or additionally, where a relatively high feed gas pressure is present (e.g., 1600 psig and higher), the demethanizer column pressure could theoretically be increased to thereby reduce residue gas compression horsepower and lower the overall energy consumption. However, the increase in demethanizer pressure is typically limited to between 450 psig to 550 psig as higher column pressure will decrease the relative volatilities between the methane and ethane components, making fractionation difficult, if not even impossible. Consequently, excess cooling is generated by the turbo-expansion from most high pressure feed gases, which heretofore known processes cannot fully utilize.
Exemplary NGL recovery plants with a turbo-expander, feed gas chiller, separators, and a refluxed demethanizer are described, for example, in U.S. Pat. No. 4,854,955 to Campbell et al. Here, a configuration is employed for ethane recovery with turbo-expansion, in which the demethanizer column overhead vapor is cooled and condensed by an overhead exchanger using refrigeration generated from feed gas chilling. Such additional cooling step condenses most of the ethane and heavier components from the demethanizer overhead, which is later recovered in a separator and returned to the column as reflux. Unfortunately, high ethane recovery is typically limited to 80% to 90%, as C2 recovery is frequently limited by CO2 freezing in the demethanizer. Therefore, the excess chilling produced from the high pressure turbo-expander cannot be utilized for high ethane recovery, and must be rejected elsewhere. However, propane refrigeration is typically required in refluxing the deethanizer in such configurations which consumes significant amounts of energy. Therefore, and with respect to feed gas having relatively high pressure and low propane and heavier content, all or almost all of the known processes fail to utilize potential energy of the feed gas.
NGL recovery processes that include CO2 removal in the NGL fractionation column are taught by Campbell et al. in U.S. Pat. No. 6,182,469. Here, a portion of the liquid in the top trays is withdrawn, heated, and returned to the lower section of the demethanizer for CO2 removal. While such configurations can remove undesirable CO2 to at least some degree, NGL fractionation efficiency is reduced, and additional fractionation trays, healing and cooling duties must be added for the extra processing steps. At the current economic conditions, such additional expenditures cannot be justified with the so realized marginal increase in ethane recovery. Still further, such systems are generally designed for feed gas pressure of 1100 psig or lower, and are not suitable for high feed gas pressure (e.g. 1600 psig or higher). Further known configurations with similar difficulties are described in U.S. Pat. Nos. 4,155,729, 4,322,225, 4,895,584, 7,107,788, 4,061,481, and WO2007/008254.
Thus, while numerous attempts have been made to improve the efficiency and economy of processes for separating and recovering ethane and heavier natural gas liquids from natural gas and other sources, all or almost all of them suffer from one or more disadvantages. Most significantly, heretofore known configurations and methods fail to exploit the economic benefit of high feed gas pressure and the cooling potential of the demethanizer, especially when the feed gas contains a relatively low C3 and heavier content. Therefore, there is still a need to provide improved methods and configurations for natural gas liquids recovery.